(1) Field
An object of an exemplary embodiment of the invention is an arrangement for supporting the stator of an electrical machine.
(2) Description of Related Art
The object of the invention is an arrangement for supporting the stator of an electrical machine according to the preamble part of claim 1.
The fundamental wave of magnetic flux creates a rotating force field in the air gap of an electrical machine, between the rotor and the stator. The spatial wavelength of this harmonic force field is 360/2 p degrees, in which p is the number of pairs of poles. The rotating force wave is a required part of electrical machine operation and cannot be substantially reduced. On the other hand, the amplitude of the force wave is so great that even a thick stator back tends to deform. This happens in spite of the fact that the elastic properties of the stator pack in a plane perpendicular to the shaft are close to the corresponding values for steel. On the other hand, the deformations of the stator have a quasi-static nature because the frequency of the characteristic form corresponding to the force wave, for example 400 Hz, is clearly higher than the excitation frequency, for example 100 Hz. The excitation frequency is the supply frequency multiplied by two.
The frame structure of large electrical machines is usually box-shaped. The frame structure is usually relatively symmetric in relation to the vertical plane containing the axial line and the sectional plane perpendicular to the axial line going through the centre point. However, the motor is usually asymmetric in relation to the horizontal plane containing the axial line. In such a structure the rotating deformation wave present in the stator creates vibration in the frame structure due to the asymmetry of the structure and support. These deformations are manifested as bearing vibration, for example.
The stator usually comprises thin plates that are attached together using back beams and welding, for example. The outermost part of the stator forms the stator back, and there are grooves in the internal part of the stator in which the windings are placed. Stator deformation can be reduced by making the stator back thicker or by using external reinforcements to stiffen the stator. However, this will increase the number of stages of manufacture, the manufacturing costs, the weight of the machine and its space requirement.
Another method of solving the problem is isolating the stator from the frame. The objective of isolation is to reduce the transmission of stator deformations to the frame and the subsequent emergence of bearing vibration. A common method for the construction of machines is to suspend the source of vibration in a flexible manner. In the case of a stator, this is impeded by the fact that the stator support must be able to bear quasi-static and dynamic loads in various situations. Such loads include gravitational force, nominal torque and short circuit torque. Furthermore, the stator pack and its suspension have a substantial effect on the characteristic frequencies and vibration behaviour of the entire motor.
The vibration problem at the second multiple of the supply frequency is particularly substantial in large two-pole asynchronous machines. Higher numbers of poles shorten the spatial wavelength of the force field, which reduces the deformations of the stator.